Present Tectonic Setting and Spatio-Temporal Distribution of Seismicity in the Apennine Belt

In previous papers, we have argued that a close connection may exist between the discontinuous northward displacement of the Adria plate and the spatio-temporal distribution of major earthquakes in the periAdriatic regions [1]-[3]. In particular, five seismic sequences are tentatively recognized in the post 1400 A.D. seismic history, each characterized by a progressive migration of major shocks along the eastern (Hellenides, Dinarides), western (Apennines) and northern (Eastern Southern Alps) boundaries of Adria. In this work, we describe an attempt at gaining insights into the short-term evolution of the strain field that underlies the migration of seismicity in the Apennine belt. The results of this study suggest that seismicity in the study area is mainly conditioned by the fact that the outer (Adriatic) sector of the Apennine belt, driven by the Adria plate, is moving faster than the inner (Tyrrhenian) belt. This kinematics is consistent with the observed Pleistocene deformation pattern and the velocity field inferred by GPS data. The spatio-temporal distribution of major shocks during the last still ongoing seismic sequence (post 1930) suggests that at present the probability of next major shocks is highest in the Northern Apennines. Within this area, we suggest that seismic hazard is higher in the zones located around the outer sector of the Romagna-Marche-Umbria units (RMU), since that wedge is undergoing an accelerated relative motion with respect to the inner Apennine belt. This hypothesis may also account for the pattern of background seismicity in the Northern Apennines. This last activity might indicate that the Upper Tiber Valley fault system is the most resisted boundary sector of the RMU mobile wedge, implying an higher probability of major earthquakes.